I buy new probably about once a year or so. Honestly because I am also a VS fanatic I try to make them last a little longer since they're expensive. I keep about 4-5 that I love on hand in basic colors like black, white, and nude, and then toss in one or two fun colors during the semi-annual sale every year to spice thing up a bit.
I usually toss when the padding starts looking unsmoothe or the straps start to lose their elasticity. I'm in love with the VS Sexy balconet bras. They're perfect for that extra oomph that a smaller cup size needs, and they last a long time. I have one that I'm just now getting ready to toss and I've had it for at least two years. The cups still look great, but the straps are worn out.
I usually toss when the padding starts looking unsmoothe or the straps start to lose their elasticity. I'm in love with the VS Sexy balconet bras. They're perfect for that extra oomph that a smaller cup size needs, and they last a long time. I have one that I'm just now getting ready to toss and I've had it for at least two years. The cups still look great, but the straps are worn out.